Bad in bed?? why?

Indi
I know this sounds a bit shallow but I'm honestly so curious. Everyone has a first time and has to learn at some point. So if sex is supposed to come naturally, why are some people not so great in bed?? 
I understand in some aspects no communication or lack there of can make for a sad day in the sack but what about the ones in love? The ones who want to actually satisfy their partners? Can sex be taught? Is it worth having a talk about it? What are your thoughts? And have you slept with someone who just couldn't quite get it?